This invention relates generally to alignment systems and more particularly to ensuring that each X-ray mask in a set of such masks is properly aligned with respect to a substrate to be exposed by X-ray lithography.
Soft X-ray lithography (see Soft X-Ray Lithographic Apparatus and Process, Smith et al., U.S. Pat. No. 3,743,842, July 3, 1973) has been shown to be an effective and convenient means of replicating high resolution patterns. This capability has been demonstrated by the replication in a polymer recording film of a repetitive grating with linewidths less than 180 nanometers. Soft X-ray lithography is also an effective and convenient means of fabricating microelectronic devices. In many fabrication procedures, several separate X-ray masks must be exposed on the same substrate, and the patterns produced by these several masks must be precisely superimposed. In many cases, extensive processing of the substrate occurs in between each X-ray exposure. Thus, a method for aligning the several X-ray masks with respect to a substrate is required. Optical alignment methods have been used successfully in the fabrication of MOS transistors (S. E. Bernacki and H. I. Smith, "Fabrication of Silicon MOS Devices Using X-Ray Lithography," IEEE Trans. ED, vol. ED22, pp. 421-428, July, 1975), but such optical means require that the soft X-ray mask be optically transparent, at least in certain regions. Moreover, in X-ray lithography it is desirable to have a finite separation between mask and substrate so as to avoid problems of mask and substrate damage. Separations of 10 to 30 micrometers are considered optimum, and at such separations, optical diffraction would prevent the achieving of the sub-micrometer superposition precision desired for X-ray lithography. A soft X-ray mask alignment system (see "Soft X-Ray Mask Alignment System," Smith et al., U.S. Pat. No. 3,742,229) has been patented that is based on the detection of soft x-rays transmitted through matching registration means on the X-ray mask and the substrate. This system does not require that the X-ray mask have optically transparent regions, and it can provide sub-micrometer superposition precision at mask-to-substrate separations of several micrometers. However, this system requires that the substrate be transparent to soft X-rays in the region behind the registration means on the substrate, so that X-rays can pass through the substrate to a soft X-ray sensor. This requirement that the substrate be transparent to soft X-rays in certain regions is a significant impediment to the commercial application of X-ray lithography. In the case of silicon substrates, in order to make certain regions soft X-ray transparent, the silicon in these regions must be reduced in thickness to about 30 micrometers. In the case of higher atomic number substrates, such as gallium arsenide or garnet, it would be necessary to drill holes entirely through the substrate and to support the substrate's registration means on thin, fragile membranes spanning the holes.
It is therefore an object of this invention to provide an alignment system for a soft X-ray lithographic apparatus which is capable of submicrometer superposition precision, and thus compatible with the high resolution capability of soft X-ray lithography.
It is a further object of this invention to provide such an alignment system which will function when the separation distance between the soft X-ray mask and the substrate exceeds several micrometers.
It is a further object of this invention to provide such an alignment system which will function when the substrate is opaque to soft X-rays.
It is a further object of this invention to provide such an alignment system which provides for precise superposition of each mask in a set of masks with a substrate to be exposed.